1. Field
This patent specification relates to a relative position detection device and a detector for a rotary body and an image forming apparatus including the relative position detection device.
2. Description of the Related Art
Various techniques of displacement measurement using image sensors have been proposed and used in displacement measurement devices and optical mouse devices.
In a conventional displacement measurement device, data of an image captured by an image sensor is compared to previously captured image data to calculate a moved position of the image. For example, the previously captured image is shifted one pixel by one pixel to calculate the autocorrelation between the currently captured image and the previously captured image. The position having the highest correlation coefficient is recognized as the current position of the image.
Such a displacement measurement method is convenient because, unlike a common encoder, a detector can independently measure displacement from the image pattern of a detected surface without the use of a reference scale. In addition, compared with an encoder using a scale in which data is updated by incrementing on a signal edge, position data is updated for each sample, which leads to reduction in time and improves real-time capability.
However, the above-described technique has the following difficulties: (1) position detection using correlation between images requires calculation of correlation coefficients for every pixel, and thus the number of calculations is large and real-time measurement is difficult; (2) to obtain a cumulative travel distance, each relative travel distance obtained by each sample is added, and thus measurement errors accumulate through the calculation. Furthermore, to perform higher accuracy position measurement, a larger number of pixels are used, which increases the number of calculations.
In the case of a conventional rotary encoder using images of inclined lines that traverse a line sensor in the longitudinal direction of the line sensor in accordance with the rotation of a disk, the angle of the rotation of the disk is detected by the resolution power based on pixel unit of the line sensor. Therefore, even a line sensor having a large pixel pitch detects the rotation at a higher resolution. The disk also has a circular pattern that is formed in the direction of rotation of the disk. The line sensor reads the circular pattern on the disk to detect and correct displacement of the center of the disk. Consequently, movement in the direction of this displacement is not detected as movement in the direction of rotation of the disk.